Numerical study of laminar starting forced plumes at high Schmidt number
Résumé
The knowledge of the dispersion and mixing mechanisms of hydrogen in an air-filled enclosure is an important safety issue for all hydrogen-based systems. Previous work on the concentration build-up in an enclosure include the theoretical studies, experimental studies and numerical studies. Significant differences concerning the concentration field have been found between experimental and numerical results at low injection flow rates. Detailed reference data for the assessment of turbulence models are therefore necessary. This work deals with the convection of a binary mixture of glycerol-water in an isothermal enclosure at constant pressure. Mass diffusion is very weak due to high Schmidt number of the fluids (beyond 10^3). Injection conditions cover a wide range from buoyancy-dominated to momentum-dominated regimes. The unsteady laminar model is solved by integrating the coupled conservation equations of the mixture mass, species mass and momentum, completed by the variation laws for the physical properties. Time advancement is performed using a fractional time step method and spatial discretization is performed with finite volume method on a staggered mesh, which are both second-order accurate. We aim at validating the numerical method with the experimental results of Rogers and Morris (2009) on the starting period of laminar liquid forced plumes. The injected fluid develops a structure consisting of an almost cylindrical conduit under a well-defined head, evolving in time. After a short initial period of acceleration the plume attains a constant ascent velocity, which is the increasing rate of the plume height. For Reynolds and Richardson numbers varying in the range [0.3-15] a good agreement with the experiment has been found concerning the ascent velocity as well as the head shape. A modified scaling law of the ascent velocity versus the Reynolds number is proposed.
Domaines
Mécanique [physics]
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